Question

A cistern has two taps which fill it in 12 minute and 15 minutes respectively. There is also a waste in the cistern. When all the pipes are opened empty cistern is full 20 minutes. How long waste pipe take to empty a full cistern?

• A. 8 minutes
• B. 10 minutes
• C. 12 minutes
• D. 16 minutes.

Answer 1

The correct option is B 10 minutes

Work done by waste pipe in 1 minutes.
$=(\frac{1}{12}+\frac{1}{15})=\frac{1}{20}=(\frac{3}{20}-\frac{1}{20})=\frac{1}{10}$
$\therefore$ Waste pipe can empty the cistern in 10 minutes.